1. Field of the Invention
The present invention relates generally to a system for monitoring and controlling current flow between an alternating current (ac) power source and a utility or utility network and more particularly, to a control system for controlling the amount and magnitude of direct current (dc) and even harmonics current injected with ac supplied to a utility from an ac power source, such as a distributed generator.
2. Description of the Related Art
Distributed generators, particularly non-dispatchable generators, are increasingly being connected to utility grids to allow these remotely located generators to contribute electrical power to the utility rid to meet power demands and to meet electric consumers demands for alternate sources of power. Common distributed generators utilize dc power sources, such as fuel cells, batteries, photovoltaics (i.e., solar power), and wind power (when rectified) to produce the power to be supplied to the utility grid. To provide ac current to the utility grid, a power converter, e.g., dc/ac inverters, is typically connected between the dc power source and the utility grid. When the included power converter is static, the power converter is typically designed to operate as an ac current source.
Recently, the Institute of Electrical and Electronic Engineers (IEEE) enacted IEEE Standard 929-2000 which imposes limits on the magnitude of harmonic currents and dc currents that may be injected into a utility by a distributed generator or from a utility customer's premise. These limits are much stricter than previous limits and compliance with these limits is a significant concern within the distributed energy industry. Particularly, these new standards state that the amount of dc current injected into the utility by the distributed generator cannot exceed 0.5 percent of the distributed generator's output current rating. Further, the new standards for injected even harmonics require that the second harmonic current be kept less than 1 percent of the output current rating of the distributed generator or ac power source. These standards require that the distributed generator function as a substantially sinusoidal ac current source with a low harmonic content (as specified by IEEE 929).
Stricter limits were included in the new IEEE standard because small amounts of dc current injection and/or higher levels of even harmonic current can quickly and seriously damage utility equipment. The type of equipment most affected are step-down transformers which lower the voltage from the utility power lines to voltages suitable for use in electric consumer residences and in business facilities. To better understand the difficulties of meeting these standards, it may be helpful to understand how dc current injection relates to or effects even harmonics. When dc current is injected into a transformer, regardless of whether the current comes from the primary or secondary, the transformer draws even harmonics from the side of the transformer supplied by a voltage source. It takes only a small amount of dc current injection to cause the transformer to exceed the IEEE 929 limits for even harmonics. In a transformer isolated distributed generator dc current injected into the transformer from the power converter side is blocked by the transformer from passing or coupling through to the utility side of the transformer. In this regard, a voltage transformer can be used to protect the utility and its equipment from dc current injection, however, the voltage transformer by itself does not solve the problem of large even harmonic amplitudes which the utility and its transformers will see. Additionally, another, more subtle problem with the use of transformers to isolate distributed generators and connected power converters is that a power converter, such as an inverter, can become or appear to be a source of even harmonics when dc current is injected into the transformer from the utility side, even if the power converter itself is not a source of dc current injection.
The distributed power generation industry has tried, with only limited success, a number of approaches to controlling dc current injection into utilities, but none of the approaches provides a method of controlling even harmonics that satisfies the new and stricter IEEE standards for protecting utility equipment. One control approach used by manufacturers in the industry is to include a voltage transformer in the output circuit of their power generator downstream of a power converter. Under the previous standards, this approach was useful for addressing the dc current injection problem as the transformer blocked the dc current output by the power converter. Unfortunately, as discussed above, the transformer draws even harmonics from the utility at magnitudes that exceed the new IEEE standards even at relatively low levels of dc current injection.
Another industry control approach is to embed a magnetic field sensor into the transformer to measure dc current flux and adjust the power converter, i.e., inverter, current output to zero the measured dc flux. This technique was used in the mid-1980s, but it proved expensive to embed magnetic field sensors into transformers. Additionally, such sensors have inherent problems with drift that develops over time. Drift in the measurement of a dc current generates inaccuracies in attempting to control or adjust that dc current to zero magnitude.
Some efforts were made to control dc current injection by measuring the dc injection current with a resistive shunt and amplifier and then using this measurement to adjust the power converter to obtain a desirable current output. However, since the dc current has to be controlled per the new IEEE standards to less than 0.5 percent of the ac current output from the generator, it is difficult to filter the relatively small dc current from the relatively large ac current signal. Additionally the resistive shunt is typically at a potential other than ground which leads to problems in measuring a small dc voltage in the presence of a substantial common mode voltage. As with the magnetic field sensor approach, even small offsets or errors in the measurement of dc current makes it unlikely that the control system will be able to zero out the dc current to satisfy the new IEEE dc current injection standards.
Yet another control system places large capacitors in series between the power converter connected to the generator and the transformer. The capacitors are effective in blocking dc current from getting into the transformer (and, thereby, avoiding drawing in even harmonics) but at a significant cost. A half bridge inverter is a topology that places the transformer in series with the dc blocking and storage capacitors but is not necessarily the most cost effective approach.
Prior to the adoption of the new IEEE standard, many manufacturers in the industry tried to address the injection problems by relying upon the use of high quality components with minimal drift and accurate microprocessor control of the power converter waveform to minimize dc current injection into the isolating transformer. Unfortunately, although dc current injection could be relatively effectively controlled in this manner even harmonic current injection is typically not kept within the new, stricter IEEE 929 limits.
Consequently, there remains a need for an improved control system for use with ac power sources, such as distributed generators, connected to utilities and/or utility grids to maintain dc current injection and even harmonic current injection below specified levels to minimize the risks of damaging utility equipment and preferably that maintains levels below the newly enacted IEEE 929 standard.